Antigen-presenting cells that activate helper T cells must express which one of the following on their surfaces?

Correct Answer: D

Rationale: The correct answer is D: class II MHC antigens. Antigen-presenting cells activate helper T cells by presenting antigens on their surfaces via class II MHC molecules. These molecules bind to the T cell receptor, triggering the immune response. IgE (choice A) is not involved in activating helper T cells. Gamma interferon (choice B) is a cytokine produced by T cells but is not directly involved in antigen presentation. Class I MHC antigens (choice C) present antigens to cytotoxic T cells, not helper T cells.

Correct Answer: A

Rationale: The correct answer is A because the increasing hemagglutination inhibition titer from 10 on January 18 to 320 on February 20 indicates a rise in antibody levels, which is consistent with the patient being infected with influenza A virus. This suggests that the patient was ill with influenza A on January 15, as it takes time for antibody levels to rise in response to an infection. Choice B is incorrect because the highest titer was on February 20, which indicates peak antibody response, not the onset of illness. Choice C is incorrect as the increasing titer levels suggest an active infection. Choice D is incorrect as the titer results are indicative of an immune response to an infection, not an autoimmune disease.

Correct Answer: B

Rationale: The correct answer is B: Both cell receptors and antibodies are highly specific in their actions. Cell receptors recognize and bind to specific molecules, triggering cellular responses. Antibodies are produced by the immune system to specifically target and neutralize foreign pathogens. This specificity is crucial for their functions. A, C, and D are incorrect: A: Cell receptors and antibodies are not produced by nerve cells. Cell receptors are typically found on the surface of various cell types, while antibodies are produced by B cells in the immune system. C: Cell receptors and antibodies do not slow the rates of chemical reactions. Their primary function is not to catalyze or inhibit chemical reactions. D: Cell receptors and antibodies are not directly involved in digestion. Digestion primarily involves enzymes and other digestive processes within the gastrointestinal tract.

Correct Answer: D

Rationale: Rationale for Answer D: 1. Scientific studies show higher allergies in cow's milk-fed babies. 2. Breast milk is the alternative in this scenario. 3. Breast milk is generally considered hypoallergenic. 4. Breast milk contains fewer potential allergens. 5. Thus, breast milk most likely contains fewer allergy triggers. Summary: A: Overgeneralization, not all infants react to cow's milk. B: Absolute statement, breast milk is not 100% effective against all allergies. C: Contradicts scientific findings, relationship exists between cow's milk and allergies.

Correct Answer: B

Rationale: The correct answer is B because early manifestations of AIDS include persistent fever, swollen glands, diarrhea, weight loss, and fatigue due to the immune system being compromised by HIV. Choice A is incorrect because HIV leads to a decreased, not elevated, white blood cell count. Choice C is incorrect as energy levels typically decline with AIDS progression. Choice D is incorrect as dry and scaly skin is not a common early symptom of AIDS.